-
Based on the statistical analysis of eastward-propagating MCSs over the second-step terrain along YHRV (Yang et al., 2019, 2020), this study investigates the formation and development of a typical type of long-lived eastward-propagating MCS. We performed semi-idealized sensitivity testing using a numerical simulation from eight representative MCS cases. The conclusions are as follows.
First, eight eastward-propagating MCSs were selected based on similarities in their synoptic weather patterns and convection initiation conditions. Next, composite meteorological fields from the eight MCSs are used as initial and boundary conditions for WRF simulation. The semi-idealized simulation successfully reproduces the formation of convection over the second-step terrain, its eastward propagation, its merger with pre-existing convection in the downstream areas, and the associated precipitation pattern. Results from the CNTL experiment indicate that the complete evolution of the MCS and its related MCV includes five stages: the MCS formation stage (1330–1530 LST), the propagation stage over the second-step terrain (1530–1900 LST), the MCV formation stage (1900–2300 LST), the maintenance stage (2300–0900 LST) and the dissipation stage (0900–1400 LST). Convection forms in regions with southwesterly winds in the low-to-midtroposphere commonly observed downstream of a 500 hPa shortwave trough. In this case, the shortwave was located east of the TP and on the northwest periphery of the WPSH, where abundant warm and moist air provides favorable moisture conditions for convection initiation. Convection initiation results from the release of CAPE triggered by the low-level convergence of southeasterly and northeasterly winds. After initiation, convection gradually propagates eastward under the influence of westerlies in the middle troposphere. During this time, moist convection develops and intensifies into MCS due to the low-level convergence and unstable stratification in the lower troposphere.
Figure 13 shows the conceptual model of the impact of an eastward-propagating MCS over the second-step terrain on the evolution of an MCV over downstream regions. While an MCS propagates out of the eastern edge of the second-step terrain and merges with the convection systems over the plains, the corresponding wind perturbation intensifies into a vortex at 850 hPa. The mesoscale vortex moves eastward, and the enhanced southwesterlies on the southeastern periphery of this vortex gradually promote enough local wind convergence to develop a local vortex (V1) at 850 hPa. This intensified local vortex merges with the leeside vortex at 925 hPa (V2) and finally develops into a mature MCV. The presence of enhanced nocturnal LLJ enables the merged convection to develop further. The MCV then intensifies and moves eastward and subsequently merges with the continuously strengthening vorticity centers on its eastern (downstream) side. At this time, the MCV reaches the mature stage with notable precipitation along its southern flank where convergence associated with the LLJ is most strongly focused.
Figure 13. The conceptual model of the impact of an eastward-propagating MCS over the second-step terrain on the evolution of MCV over the downstream regions, including stages: eastward-propagation of MCS1 out of the second-step terrain, merger with the local convection system, formation and maintenance of MCV.
Results from the sensitivity experiment with diabatic heating in the formation region turned off indicate that MCSs do not form and move eastward over the second-step terrain when no diabatic heating is available. In the absence of eastward-propagating MCSs, convective and mesoscale vortices still exist in the plains to the east of the second-step terrain along YHRV, but the vortex strength and precipitation intensity weaken markedly. This result indicates that the eastward movement of these long-lived MCSs has a significant impact on the development and enhancement of convection and vortices in the downstream areas.
Based on a semi-idealized simulation from composite fields, the present study reveals the formation, development, and impact on downstream systems of a typical type of eastward-propagating MCSs. Future studies will analyze real cases and simulate them to gain a deeper understanding of the impact of second-step terrain along YHRV on the initiation of MCSs and the relevant mechanisms related to mesoscale vortices induced by eastward-propagating MCSs.
Acknowledgements. This article is dedicated to Prof. Fuqing ZHANG, who greatly contributed to our long-term international cooperation on mesoscale meteorology. We sincerely appreciate Prof. Fuqing ZHANG for all the suggestions, discussions, and help regarding the relationship between MCSs and mesoscale vortexes east of the second-step terrain in China. This research was supported by the National Key R&D Program of China (Grant No. 2018YFC1507200) and the National Natural Science Foundation of China (Grant No. 41975057).
Case number Formation time Termination time Formation longitude (oE) Formation latitude (oN) Formation height (m) 1 2000062720 2000062822 107.19 31.77 545.05 2 2001062808 2001062904 112.46 33.75 639.05 3 2002062207 2002062307 112.86 32.30 173.69 4 2016062217 2016062320 108.54 32.83 766.44 5 2003070110 2003070203 107.96 33.13 597.88 6 2006062108 2006062205 111.34 31.06 612.49 7 2010081314 2010081405 107.93 32.75 1207.59 8 2012070810 2012070901 109.54 32.62 849.81 Notes: time is denoted in yyyymmddhh, i.e., 2000062620 means 2000 UTC 27 June 2000. Table 1. Time and location of the formation and termination of eight MCSs.
Case number | Formation time | Termination time | Formation longitude (oE) | Formation latitude (oN) | Formation height (m) |
1 | 2000062720 | 2000062822 | 107.19 | 31.77 | 545.05 |
2 | 2001062808 | 2001062904 | 112.46 | 33.75 | 639.05 |
3 | 2002062207 | 2002062307 | 112.86 | 32.30 | 173.69 |
4 | 2016062217 | 2016062320 | 108.54 | 32.83 | 766.44 |
5 | 2003070110 | 2003070203 | 107.96 | 33.13 | 597.88 |
6 | 2006062108 | 2006062205 | 111.34 | 31.06 | 612.49 |
7 | 2010081314 | 2010081405 | 107.93 | 32.75 | 1207.59 |
8 | 2012070810 | 2012070901 | 109.54 | 32.62 | 849.81 |
Notes: time is denoted in yyyymmddhh, i.e., 2000062620 means 2000 UTC 27 June 2000. |